Many relatively large turbine engines, including turbofan jet engines, may use an air turbine starter (ATS) to initiate their rotation. The ATS is mounted by the jet engine, much as a starter for an automobile is located by the automobile's engine. The ATS may be coupled to a high pressure fluid source, such as compressed air, which impinges upon the turbine wheel in the ATS causing it to rotate at a relatively high rate of speed. The ATS includes an output shaft that is coupled, perhaps via one or more gears, to the jet engine. The output shaft rotation in turn causes the jet engine to begin rotating. The applicant for the present invention, Honeywell International, Inc., has for years successfully designed, developed and manufactured ATSs.
The ATS turbine wheel output shaft may be rotationally mounted within a housing using one or more bearing assemblies. The bearing assemblies, as well as the above noted gears, may be supplied with a lubricant, such as oil. Thus, the ATS may be mounted within a housing that is divided into at least two sections, the turbine section and the output section. The turbine section houses the turbine wheel and includes one or more passages through which the high pressure fluid source passes and impinges upon the turbine wheel, causing the turbine wheel to rotate. The output section, or gearbox, may house the turbine wheel output shaft, the gears, the bearing assemblies, and various other mechanical devices that utilize a lubricant. A seal assembly may be provided between the turbine section and output section of the ATS to substantially inhibit the lubricant used in output section from leaking out of the output section into the turbine exhaust section.
The seal assembly may be a face seal that includes a rotor, a stator, and a seal case. The rotor is mounted on the turbine wheel shaft and, thus, rotates with the turbine shaft, and has an axially facing flange, or sealing face, that extends radially away from shaft. The seal case is mounted to the ATS housing in the turbine section and surrounds the turbine wheel output shaft. The stator is housed within the seal case and sealingly engages the axially facing flange of the rotor. The rotor and stator flat annular faces sealingly engage under a biasing force imposed by a biasing mechanism in the seal case.
Face seal stators with carbon faces are known to be used as seals in engines, including air-turbine engines and air turbine aircraft engine starters. Carbon-stator face seals encounter high-temperature loads caused by friction between the carbon stator sealing face and the rotor face, which may be metal. Heat may cause the oil on the seal rotor and stator to solidify into coke as a result of the high temperatures at the face. The coke accumulations may compromise face seal performance and limit face seal life. Compromise of a face seal can result in sufficient loss of lubrication to the bearings, gears, and other lubricated components in the air turbine starter gearbox to cause damage to these components. It should be appreciated that ATS's with the above design are nonetheless safe for their intended use.
Carbon-stator face seals may additionally incorporate other technologies such as film-riding face geometries (Rayleigh, Spiral, and wave designs) as discussed in NASA/TM-1998-206961 AVT-PPS Paper No. 11 “Advanced Seal Technology Role in Meeting Next Generation Turbine Engine Goals”. Various film-riding echnologies are known in the art, and generally include shaped configurations of the sealing surfaces of either the stator or the rotor. The shaping of the sealing surface is specific to the task of maintaining a film of a fluid between the stator and rotor sealing surfaces to minimize friction while maintaining a seal. The fluid used may be, for example, air, oil, or an air-oil mixture.
Hence, there is a need for a seal assembly that reduces the rate and likelihood of coke accumulation between the stator sealing face and the seal rotor face, thereby reducing the likelihood of loss of lubrication to rotating components within the starter gearbox. The present invention addresses this need.